When building roadways, parking lots and the like, for example, paving machines may be used to deposit paving material, such as asphalt, on a paving surface to create a flat, consistent surface over which vehicles will travel. A paving machine at the construction site, such as an asphalt paver, is generally a state-of-the art self-propelled construction machine designed to receive, convey, distribute, profile and partially compact the asphalt material. The paving machine accepts asphalt material that is heated to an appropriate temperature for flow and even spreading into a receiving hopper at the front of the machine. The asphalt material in the hopper is conveyed to the rear of the machine with parallel slat conveyors or other types of conveyors positioned at the bottom of the hopper. The asphalt material conveyed from the hopper is distributed along the width of an intended ribbon or mat by means of two opposing screws or spreading conveyors or augers, and a free-floating screed profiles and compacts the asphalt material into a mat on the paving surface.
The operation of the paving machine and its components may be manually controlled by the operator(s) to dispense the asphalt material and create the mat on the paving surface. In many paving machines, systems are provided to automate and control the paving process for consistent operation of the paving machine for laying a uniform mat on the paving surface without defects compromising the integrity and longevity of the mat. The automation systems may include control over the speed of the paving machine, operation of the conveyors and augers to distribute the asphalt material, and vertical positioning and temperature control of the screed. The control settings may be established during an initial setup process for a paving job, such as the paving of a stretch of a highway or the paving of a parking lot.
During the paving process, the screed must be heated so that the hot paving material does not stick to the bottom surface as the screed passes over and compacts the mat. The screed has a plurality of electrical heating elements that heat the screed to the necessary temperature for paving. The heating elements receive electricity via relays from a generator of the paving machine. In most current paving machines, a power source, such as a gas or diesel internal combustion engine, has an output shaft driving a pump drive. The pump drive in turn drives multiple pumps and/or motors providing pneumatic, hydraulic and mechanical power to the various systems of the paving machine. One of the pumps or motors drives the generator supplying electricity to the screed heating elements. Most generators used in these applications provide alternating current (AC) power with frequencies that vary as the speed of the output shaft of the power source varies. An additional rectifier circuit or other signal conditioning arrangement is provided to convert the output of the generator to AC power having an approximately constant frequency.
The use of the string of interconnected components as described results in power losses and reduced efficiency that increase the cost of operating the paving machine. Some paving machine arrangements provide a more direct connection between a power source and a generator. For example, U.S. Pat. Appl. Publ. No. 2010/0296866, published on Nov. 25, 2010 and entitled, “Paver and Method,” discloses a generator that is mounted either at the power take-off gear for the road paver's pumps or at a suspension in the chassis of the road paver, or at a console of the combustion engine of the road paver. The generator may be driven by a permanent drive connection, such as a belt drive or a drive shaft. However, the publication does not teach any alternate connection to heating elements of the road paver than that described above.
During operation of the paving machines having the screed heating arrangements described above, the screed temperature is maintained at the desired level as the mat is laid on the paving surface. Typically, each of the screed heating elements has a corresponding circuit breaker that trips in the event of a ground fault to prevent damage to the components. When a ground fault occurs, the circuit breaker trips without an indication being given to the operator that the corresponding heating element is not receiving power. The paving machine continues to lay the mat on the paving surface as the portion of the screed cools, and the operator may not identify the failure of heating element until portions of the mat are adversely affected by contact with the cooled portion of the screed.
In view of the inefficiencies and performance risks present in providing power to the screed heating elements in the present paving machines, a need exists for improved electrical power generation and transmission to the screed heating elements, and improved handling of ground fault situations to minimize the adverse affects on the mats being laid by the paving machines.